The present invention relates to a rubber composition, more particularly, to a rubber composition capable of giving rubber vulcanizates having excellent heat resistance and comprising a binary copolymeric rubber of ethylene and propylene or ternary copolymeric rubber of ethylene, propylene and a diene compound as the principal ingredient and which are free from the problem of stain on the surface of the metal mold even after repeated molding of the rubber composition therein.
Usually shaped rubber articles are manufactured by molding a rubber composition in a metal mold at a high temperature under high compressive pressure to effect vulcanization of the rubber composition. A problem in this rubber molding process is that the surface of the metal mold sometimes becomes stained by repeating the molding procedure to such an extent that the rubber article molded and vulcanized in such a stained metal mold no longer has a surface gloss which greatly decreases the yield of acceptable products. Therefore, metal molds must be cleaned periodically in order to ensure acceptable quality of the rubber products molded therein. Cleaning metal molds are very time-and labor-consuming and greatly increases the manufacturing costs of the rubber articles. Accordingly, it is important that the rubber composition is so formulated that the metal mold used for molding the composition is less liable to surface stain even after repeating the molding procedure many times.
The staining of metal molds mentioned above has been a serious problem in the rubber industry for a long time but the mechanism leading to the phenomenon is not yet fully clear. Along with the recent progress in the technology of analysis, however, it is now possible to directly examine the surface of metal molds so that certain information has already been obtained on the constituents of the stain deposited on the surface of metal molds used for rubber molding. It is reported, for example, that the stain is mainly composed of a degradation product of the rubbery polymer and the aging retarder usually formulated in most rubber compositions. Such information, however, has not yet been utilized to establish a measure for solving the problem of stain on the metal molds.